This invention relates generally to improved methods and apparatus for initiating an extrusion process, and more particularly to new and improved methods and apparatus for initiating the extrusion of tubular product of decreased diameter and decreased wall thickness from a tubular workpiece of larger diameter and larger wall thickness.
As is known to those skilled in the art of extruding a tubular workpiece of larger diameter and larger wall thickness into tubular product of smaller diameter and reduced wall thickness, typically, the tubular workpiece is advanced through an annular space provided between the end of the mandrel and the opposed surface of a tapered extrusion die of decreasing diameter in the direction of extrusion, with the width of the annular space being smaller than the wall thickness of the workpiece. Upon the initiation of the extrusion process, the tubular workpiece advances relatively freely to the annular space but upon initially engaging the die surface, or being advanced into the annular space of reduced width, initial extrusion resistance is met upon the annular space being initially closed or plugged by the advancing end of the workpiece before the tubular workpiece is first advanced through the annular space and actual extrusion commenced. This initial resistance or "plugging" causes the extrusion pressure to increase virtually instantaneously from zero to some peak, which peak has been found to exceed the running or continuous extrusion pressure by 25%-100% or even infinitely upon an initial "plugging" or "jam" condition. This initial resistance, or "plugging" or "jam," occurs so suddenly, virtually instantaneously, that lubrication provided on the I.D. and O.D. of the tubular workpiece does not have an opportunity to stabilize or function and the tip of the tubular workpiece experiences cold work hardening producing redundant work which increases the difficulty of extruding the workpiece tip. Further, the increase in the temperature in the tubular workpiece flowing through the annular space or zone of deformation during actual extrusion with its attendant enhancement of the extrusion process does not have an opportunity to occur and hence the initiation of the extrusion process is further retarded. Accordingly, it has been found that this initial extrusion condition causes extrusion machine overloading, excessive design requirements for the machine with its attendant excessive cost, occasional machine broken parts, and poor quality product at the beginning of the extrusion process with its attendant waste and undesirable cost.
Various solutions to this extrusion start-up condition are known to the prior art some of which, given their specific intended purpose, have worked reasonably satisfactorily. For example, such prior art solutions are disclosed in U.S. Pat. No. 3,756,061 issued Sept. 4, 1973, U.S. Pat. No. 3,767,368 issued Oct. 23, 1973, U.S. Pat. No. 3,766,769 issued Oct. 23, 1973 and U.S. Pat. No. 3,924,429 issued Dec. 9, 1973 (all such patents issued in the name of Francis J. Fuchs, Jr., inventor). However, it has been found that there still exists a need in the extrusion art for improved methods and apparatus for initiating tubular extrusion particularly where tubular product of reduced diameter and reduced wall thickness is produced from a tubular workpiece of larger diameter and larger wall thickness.